This invention concerns distribution of multimedia files by file servers over an arbitrary network for information transfer, like a wide area network (WAN), for example the Broadband-ISDN or the Internet, or like a local area network (LAN). It is related to all time critical files which are coded with a variable amount of data over time, for instance, video files like movies, audio presentations, or teleconference presentations. In particular, the invention is directed to video-on-demand (VOD) services, as being envisaged for interactive television where the transmission quality is to remain constant despite given fluctuations in the amount of coded data, and where a continuous play-back of a demanded video is provided.
Known video file servers in such an environment serve as providers to play back compressed digital videos to clients over a telecommunication network. Those clients, on their side, need special decoders to convert the digital video signal in a television signal. These decoders comprise further a decoder buffer, and are usually implemented in a so-called Set-Top-Box. The videos are transmitted as frames of a given time duration. Commonly a predetermined constant bitrate for data file transfer is provided in order to achieve a simple management of network resources (FIG. 2). As the compression ratio and therefore the size of a compressed frame of a video depends on the respective scene content, a compressed video comprises inherently a variable bitrate. For these reasons, file servers delivering movies are confronted with the problem of an extreme time dependent variation of information amount which has to be transferred from the server to the user at a given time.
A conventional multimedia data server is disclosed in U.S. patent application Ser. No. 288525 filed on Aug. 10, 1994, by the present applicant. Thisxserver comprises a direct access storage device (DASD), a plurality of data buffers, each of said buffers corresponding to a respective time interval associated with a multimedia presentation, ahd a mass storage device for storing the multimedia presentation data. A data bus connects the mass storage with the plurality of data buffers, wherein a bus switch is provided being operated by a controller.
A Set-Top-Box for the receiving side of a multimedia communication system according to the prior art is disclosed in U.S. patent application Ser. No. 473315 filed on Jun. 7, 1995, which is also assigned to the present applicant. This device effectuates local reception and processing of multimedia programs received from a remote multimedia file server. The local Set-Top-Box control system includes a DASD adapted to buffer a predetermined number of compressed program segments received from a multimedia file server. In particular, that invention is directed to a novel formatting methodology which provides concurrent presentation and buffering of program segments received from the server for on-demand viewing of a selected multimedia program.
Further for the transmission of compressed video files over a digital telecommunication link, for instance one operating in the known Asynchronous Transfer Mode (ATM), two compression modes are currently considered: constant bitrate and variable bitrate. These modes are illustrated in FIG. 2. A respective approach to achieve a constant bitrate, is to use a special compression mechanism with a so-called xe2x80x9cclosed-loop controlxe2x80x9d in the encoder that adjusts the picture quality as a function of the encoder buffer fill level. The respective variable quantization in the compression process leads to a variable quality. The quality decreases in scenes with high image complexity or heavy motion. In contrast, a compression process with constant quantization, of course, avoids the problem of changing quality, but it produces a video file with variable bitrate over time which is difficult to manage in networks with respect to the bursty traffic due to the fluctuating rate as a function of the scene contents.
The pre-cited known techniques are published in Proceedings of the 4th Open Workshop on High Speed Networks, Brest (France), September 1994, by Marcel Graf, and entitled xe2x80x9cTraffic Shaping of VBR Video in ATM Endsystemsxe2x80x9d, which particulary concerns a method of traffic shaping, which drastically reduces the burstiness of a video stream.
A typical scenario for use of a file server coupled to a WAN is that at connection set-up time the user negotiates with the respective network provider a traffic contract that specifies the traffic the user injects to the network and the so-called xe2x80x9cQuality of Servicexe2x80x9d (QoS) the user gets from the network. On the user side a traffic shaping function shapes the outgoing traffic so as to comply with the contract by buffering the cells to be sent. Traffic shaping is a means to control the burstiness of the traffic, but the transmission of realtime data imposes further constraints on traffic shaping. The individual video frames must arrive on time at the receiver. To eliminate jitter introduced in the transmission process, the receiver buffers the incoming frames in a smoothing buffer before they are decoded. The traffic shaping function introduces additional jitter. These two types of jitter need to be eliminated.
In the above article by M. Graf the trade-off between reducing burstiness and generating additional jitter and smoothing delay is analyzed, too. The proposed solution is a fluid model for video transmission, which enables to determine the jitter and smoothing delay introduced by shaping, as well as the buffer capacity requirements at the sender side and the receiver side. Jitter smoothing is achieved by properly delaying the start of the video decoder, wherein buffer underflow must be avoided, i.e. the consumption instant of each frame must not be earlier than the arrival instant of the frame in the buffer. In the article it is shown that jitter smoothing introduces an additional delay equivalent to the maximum jitter appearing in the frame sequence. It is observed that for movies or music recordings the quality of transmission is much more important than the starting time of a respective presentation. The user is already accustomed to initial delays in this area due to the necessary operation steps when starting a video tape on a video assembly or recorded music on a tape recorder. Hereby an initial delay of up to about ten seconds can be tolerated.
A similar approach is disclosed in an conference article by Wu-Chi Feng and S. Sechrest in Proceedings of the SPIE-The International Society for Optical Engineering (1995) Vol. 2417, p. 234-242, and entitled xe2x80x9cSmoothing and buffering for delivery of prerecorded compressed videoxe2x80x9d. The authors propose a method for smoothing and buffering digital video data for delivery of pre-recorded compressed videos. The article is also concerned with the problem that transportation of compressed video data require the network to adapt to large fluctuations in bandwidth requirements if the quality of the video is to remain constant. The proposed use of a technique of averaging for smoothing video data allows for smoothing at the expense of a further source for delay.
A further prior art approach concerning multimedia video file distribution over WANs, given for example in U.S. Pat. No. 5,613,057, which is assigned to the present applicant. The invention described therexe2x80x94in contrast to the aforementioned approachesxe2x80x94intends to reduce the delay experienced by a user between the time of a presentation request and the presentation to the user, itself It is proposed to utilize two distributions sites, intermediate sites with which users interact and which combine limited storage facilities and fast communication channels for interacting with users. A main distribution site provides mass storage for files and slower and cheaper communication channels to the intermediate sites for the file distribution. The intermediate sites provide partial caching for some number of files. When a presentation request is received from a user at the intermediate site, the intermediate site determines if the file is already partially cached. If so, an interrupted viewing is immediately available at the intermediate site. If not, the intermediate site determines from the size of the file and the transfer rate between the sites the amount of time that will be required for transfer. The described file server concept is also applicable for the present invention.
Further in U.S. Pat. No. 5,808,607, which is assigned to the present applicant, a method of work scheduling in a multinode multimedia file server for a distributed computerized system is disclosed, which stores and transmits movies in digitized form. Multiple nodes are linked together in an interconnect network. Each node has a processor, buffers, output ports and communication paths with a non-volatile storage for the video data. Each node services a ste of viewers for a particular movie for a particular temporal frame. Proposed is a method for optimizing the processing of video requests, whereby the movies are stored in buffers rather than having to be retrieved from the non-volatile storage. In this approach the time scale for scheduling is about minutes, and thus quite different from the time scale underlying.
Another scheduling mechanism for video-on-demand is disclosed in U.S. Pat. No. 5,561,456, which is also assigned to the present applicant. The proposed scheduler maintains a queue of pending performance for at least one movie, and determines a maximum wait tolerance time for a longest waiting one of the pending performance requests, whereby the time scale is about minutes. When the respective stream capacities become available, rather than scheduling the movie immediately, the scheduler delays performance of the video until just prior to expiration of the maximum wait tolerance time of the longest waiting one of the pending performance requests. In the interim additional streams can join the queue.
A further approach concerning tolerance times for longest waiting is disclosed in the International (PCT) patent application Ser. No. WO 9103112 A which has been published in July 1995. By use of a program transmission optimization mechanism, especially for cable TV networks, segmented programs are transmitted in redundant sequence in accordance with a scheduling algorithm. The maximum response time corresponding to a maximum time a user need wait to commence playing a requested program is selected. The scheduling algorithm used in the transmission phase ensures that the respective receiver will receive all of the program segments in a manner that will enable continuous play-back in real time of the program at the receiving side.
The drawback of a first part of the beforehand discussed approaches is that they need scheduling algorithms executed at the time of delivery of a presentation file. Thereupon they need execution of complex algorithms to smooth the video data, also at the time of delivery of a file. The other approaches are insofar disadvantageous as they use only a fixed pre-computed schedule for delivery.
It is therefore an object of the present invention to provide a multimedia file distribution mechanism and a multimedia file server, which allow a constant quality of transmission despite of the different beforehand discussed requirements for transmission of data via a WAN. In addition, these requirements should be fulfilled automatically and without a great expense of processes run at time of delivery, and should not require further interactions by the user. A particular problem to be solved are the rate fluctuations between the transferred frames representing different scenes in video-on-demand services.
The present invention solves this problem by providing an additional time delay for the presentation of a multimedia file in order to account for the delayed arrival of frames at the receiver due to the spreading of the transmission of these frames over time. The respective delay values for different multimedia files are calculated off-line, i.e. before delivery, and the respective control parameters for transmission are appended to each of the multimedia files; and a rate control mechanism in the server. The proposed control parameters guarantee an optimal adaptation of the time delay to the existing network environment, in particular the transmission bandwidth. The proposed rate control mechanism when provided with the respective control parameters, produces the schedule for the transmission of the video file that prevents buffer underflow or overflow in the receiver. The time delay can be simply adapted to other features of the underlying network, for instance the costs of the transmission or the storage capacity on the receiving side. The initialisation of the working point of the above control parameters to the network environment like transmission rate, and/or to the user environment like the encoder buffer size, is accomplished at the time of delivery of a respective multimedia file, and can be adjusted without any interaction by the user, and is thus transparent to the user.
The relationship between additional time delay and transmission rate can be calculated, for instance, by the file server, by scanning the respective presentation files to be served. The procedure has linear complexity with the length of a respective file, and thus takes only a few minutes for typical files, due to the proposed fast algorithm. This algorithm is based on the method of smoothing the transmission rate over time of the respective file. That curve is characterized by numerous peaks due to the different information content of the different multimedia events, like action sequences of movies or complex acoustic events. Those peaks are reduced by spreading the respective data, shifting it to later time of the transfer process. Hereby the additional delay time is introduced according to the respective time shift.
In a preferred embodiment the delay/transmission data is stored at the beginning of the respective presentation file, i.e. prepended, and thus can be taken into account at the initialisation stage of the presentation.